Non-polar (Al,B,In,Ga)N quantum well and heterostructure materials and devices

ABSTRACT

A method for forming non-polar (Al,B,In,Ga)N quantum well and heterostructure materials and devices. Non-polar ( 11 {overscore ( 2 )} 0 ) a-plane GaN layers are grown on an r-plane ( 1 {overscore ( 1 )} 02 ) sapphire substrate using MOCVD. These non-polar ( 11 {overscore ( 2 )} 0 ) a-plane GaN layers comprise templates for producing non-polar (Al,B,In,Ga)N quantum well and heterostructure materials and devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of thefollowing co-pending and commonly-assigned U.S. Provisional PatentApplication Ser. No. 60/372,909, entitled “NON-POLAR GALLIUM NITRIDEBASED THIN FILMS AND HETEROSTRUCTURE MATERIALS,” filed on Apr. 15, 2002,by Michael D. Craven, Stacia Keller, Steven P. DenBaars, Tal Margalith,James S. Speck, Shuji Nakamura, and Umesh K. Mishra, which applicationis incorporated by reference herein.

This application is related to the following co-pending andcommonly-assigned United States Utility Patent Applications:

Ser. No. 10/413,691, entitled “NON-POLAR A-PLANE GALLIUM NITRIDE THINFILMS GROWN BY METALORGANIC CHEMICAL VAPOR DEPOSITION,” filed on Apr.15, 2003, by Michael D. Craven and James S. Speck; and

Ser. No. 10/413,913, entitled “DISLOCATION REDUCTION IN NON-POLARGALLIUM NITRIDE THIN FILMS,” filed on Apr. 15, 2003, by Michael D.Craven, Stacia Keller, Steven P. DenBaars, Tal Margalith, James S.Speck, Shuji Nakamura, and Umesh K. Mishra, now U.S. Pat. No. 6,900,070,issued May 31, 2005;

both of which applications are incorporated by reference herein.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with Government support by the Office of NavalResearch Grant N00014-97-C-0192. The government may have certain rightsin this invention.

1. FIELD OF THE INVENTION

The invention is related to semiconductor materials, methods, anddevices, and more particularly, to non-polar (Al,B,In,Ga)N quantum welland heterostructure materials and devices.

2. DESCRIPTION OF THE RELATED ART

(Note: This application references a number of different patents,applications and/or publications as indicated throughout thespecification by one or more reference numbers. A list of thesedifferent publications ordered according to these reference numbers canbe found below in the section entitled “References.” Each of thesepublications is incorporated by reference herein.)

Current state of the art (Al,B,In,Ga)N heterostructures and quantum wellstructures employ c-plane (0001) layers. The total polarization of aIII-N film consists of spontaneous and piezoelectric polarizationcontributions, which both originate from the single polar [0001] axis ofthe wurtzite nitride crystal structure. Polarization discontinuitieswhich exist at surfaces and interfaces within nitride heterostructuresare associated with fixed sheet charges, which in turn produce electricfields. Since the alignment of these internal electric fields coincideswith the growth direction of the c-plane (0001) layers, the fieldsaffect the energy bands of device structures.

In quantum wells, the “tilted” energy bands spatially separate electronsand hole wave functions, which reduces the oscillator strength ofradiative transitions and red-shifts the emission wavelength. Theseeffects are manifestations of the quantum confined Stark effect (QCSE)and have been thoroughly analyzed for GaN/(Al,Ga)N quantum wells. SeeReferences 1–8. Additionally, the large polarization-induced fields arepartially screened by dopants and impurities, so the emissioncharacteristics can be difficult to engineer accurately.

The internal fields are also responsible for large mobile sheet chargedensities in nitride-based transistor heterostructures. Although theselarge 2D electron gases (2DEGs) are attractive and useful for devices,the polarization-induced fields, and the 2DEG itself, are difficult tocontrol accurately.

Non-polar growth is a promising means of circumventing the strongpolarization-induced electric fields that exist in wurtzite nitridesemiconductors. Polarization-induced electric fields do not affectwurtzite nitride semiconductors grown in non-polar directions (i.e.,perpendicular to the [0001] axis) due to the absence of polarizationdiscontinuities along non-polar growth directions.

Recently, two groups have grown non-polar GaN/(Al,Ga)N multiple quantumwells (MQWs) via molecular beam epitaxy (MBE) without the presence ofpolarization-induced electric fields along non-polar growth directions.Waltereit et al. grew m-plane GaN/Al_(0.1)Ga_(0.9)N MQWs on γ-LiAlO₂(100) substrates and Ng grew a-plane GaN/Al_(0.15)Ga_(0.85)N MQW onr-plane sapphire substrates. See References 9–10.

Despite these results, the growth of non-polar GaN orientations remainsdifficult to achieve in a reproducible manner.

SUMMARY OF THE INVENTION

The present invention describes a method for forming non-polar(Al,B,In,Ga)N quantum well and heterostructure materials and devices.First, non-polar (11{overscore (2)}0) a-plane GaN thin films are grownon a (1{overscore (1)}02) r-plane sapphire substrate using metalorganicchemical vapor deposition (MOCVD). These non-polar (11{overscore (2)}0)a-plane GaN thin films are templates for producing non-polar(Al,B,In,Ga)N quantum well and heterostructure materials and devicesthereon.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a flowchart that illustrates the steps of a method for formingnon-polar (Al,B,In,Ga)N quantum well and heterostructure materials anddevices according to a preferred embodiment of the present invention;

FIG. 2 illustrates the photoluminescence (PL) spectra of 5-perioda-plane In_(0.1)GaN/In_(0.03)GaN MQW structures with nominal well widthsof 1.5 nm, 2.5 nm, and 5.0 nm measured at room temperature;

FIG. 3 illustrates the PL spectra of an a-planeIn_(0.03)Ga_(0.97)N/In_(0.1)Ga_(0.9)N MQW structure with a nominal wellwidth of 5.0 nm measured for various pump powers;

FIG. 4( a) shows a 2θ-ω x-ray diffraction scan of the 10-periodAl_(0.4)Ga_(0.6)N/GaN superlattice, which reveals clearly definedsatellite peaks; and

FIG. 4( b) illustrates the PL spectra of the superlattice characterizedin FIG. 4( a).

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

The purpose of the present invention is to provide a method forproducing non-polar (Al,B,In,Ga)N quantum well and heterostructurematerials and devices, using non-polar (11{overscore (2)}0) a-plane GaNthin films as templates.

The growth of device-quality non-polar (11{overscore (2)}0) a-plane GaNthin films on (1{overscore (1)}02) r-plane sapphire substrates via MOCVDis described in co-pending and commonly-assigned U.S. Provisional PatentApplication Ser. No. 60/372,909, entitled “NON-POLAR GALLIUM NITRIDEBASED THIN FILMS AND HETEROSTRUCTURE MATERIALS,” filed on Apr. 15, 2002,by Michael D. Craven, Stacia Keller, Steven P. DenBaars, Tal Margalith,James S. Speck, Shuji Nakamura, and Umesh K. Mishra, as well asco-pending and commonly-assigned U.S. Utility patent application Ser.No. 10/413,691, entitled “NON-POLAR A-PLANE GALLIUM NITRIDE THIN FILMSGROWN BY METALORGANIC CHEMICAL VAPOR DEPOSITION,” filed on Apr. 15,2003, by Michael D. Craven and James S. Speck, both of whichapplications are incorporated by reference herein.

The present invention focuses on the subsequent growth of (Al,B,In,Ga)Nquantum wells and heterostructures on the (11{overscore (2)}0) a-planeGaN layers. The luminescence characteristics of these structuresindicate that polarization-induced electric fields do not affect theirelectronic band structure, and consequently, polarization-freestructures have been attained. The development of non-polar(Al,B,In,Ga)N quantum wells and heterostructures is important to therealization of high-performance (Al,B,In,Ga)N-based devices which areunaffected by polarization-induced electric fields.

Potential devices to be deposited on non-polar (11{overscore (2)}0)a-plane GaN layers include laser diodes (LDs), light emitting diodes(LEDs), resonant cavity LEDs (RC-LEDs), vertical cavity surface emittinglasers (VCSELs), high electron mobility transistors (HEMTs),heterojunction bipolar transistors (HBTs), heterojunction field effecttransistors (HFETs), as well as UV and near-UV photodetectors.

Process Steps

FIG. 1 is a flowchart that illustrates the steps of a method for formingnon-polar (Al,B,In,Ga)N quantum well and heterostructure materials anddevices according to a preferred embodiment of the present invention.The steps of this method include the growth of “template” (11{overscore(2)}0) a-plane GaN layers, followed by the growth of layers withdiffering alloy compositions for quantum wells and heterostructures.

Block 100 represents loading of a sapphire substrate into a vertical,close-spaced, rotating disk, MOCVD reactor. For this step, epi-readysapphire substrates with surfaces crystallographically oriented within+/−2° of the sapphire r-plane (1{overscore (1)}20) may be obtained fromcommercial vendors. No ex-situ preparations need be performed prior toloading the sapphire substrate into the MOCVD reactor, although ex-situcleaning of the sapphire substrate could be used as a precautionarymeasure.

Block 102 represents annealing the sapphire substrate in-situ at a hightemperature (>1000° C.), which improves the quality of the substratesurface on the atomic scale. After annealing, the substrate temperatureis reduced for the subsequent low temperature nucleation layerdeposition.

Block 104 represents depositing a thin, low temperature, low pressure,nitride-based nucleation layer as a buffer layer on the sapphiresubstrate. Such layers are commonly used in the heteroepitaxial growthof c-plane (0001) nitride semiconductors. In the preferred embodiment,the nucleation layer is comprised of, but is not limited to, 1–100nanometers (nm) of GaN deposited at approximately 400–900° C. and 1 atm.

After depositing the nucleation layer, the reactor temperature is raisedto a high temperature, and Block 106 represents growing the epitaxial(11{overscore (2)}0) a-plane GaN layers to a thickness of approximately1.5 μn. The high temperature growth conditions include, but are notlimited to, approximately 1100° C. growth temperature, 0.2 atm or lessgrowth pressure, 30 μmol per minute Ga flow, and 40,000 μmol per minuteN flow, thereby providing a V/III ratio of approximately 1300). In thepreferred embodiment, the precursors used as the group III and group Vsources are trimethylgallium and ammonia, respectively, althoughalternative precursors could be used as well. In addition, growthconditions may be varied to produce different growth rates, e.g.,between 5 and 9 Å per second, without departing from the scope of thepresent invention.

Upon completion of the high temperature growth step, Block 108represents cooling the epitaxial (11{overscore (2)}0) a-plane GaN layersdown under a nitrogen overpressure.

Finally, Block 110 represents non-polar (Al,B,In,Ga)N layers, withdiffering alloy compositions and hence differing electrical properties,being grown on the non-polar (11{overscore (2)}0) a-plane GaN layers.These non-polar (Al,B,In,Ga)N layers are used to produce quantum wellsand heterostructures.

The quantum wells employ alternating layers of different bandgap suchthat “wells” are formed in the structure's energy band profile. Theprecise number of layers in the structure depends on the number ofquantum wells desired. Upon excitation, electrons and holes accumulatein the wells of the conduction and valence bands, respectively.Band-to-band recombination occurs in the well layers since thedensity-of-states is highest at these locations. Thus, quantum wells canbe engineered according to the desired emission characteristics andavailable epitaxial growth capabilities.

The nominal thickness and composition of the layers successfully grownon the non-polar (11{overscore (2)}0) a-plane GaN layers include, butare not limited to:

8 nm Si-doped In_(0.03)GaN barrier

1.5, 2.5, or 5 nm In_(0.1)GaN well

Moreover, the above Blocks may be repeated as necessary. In one example,Block 110 was repeated 5 times to form an MQW structure that was cappedwith GaN to maintain the integrity of the (In,Ga)N layers. In thisexample, the layers comprising the MQW structure were grown via MOCVD ata temperature of 825° C. and atmospheric pressure.

The luminescence characteristics of this structure indicate thatpolarization-induced electric fields do not affect the band profiles,and the quantum wells can be considered polarization-free. For example,FIG. 2 illustrates the photoluminescence (PL) spectra of 5-perioda-plane In_(0.1)GaN/In_(0.03)GaN MQW structures with nominal well widthsof 1.5 nm, 2.5 nm, and 5.0 nm measured at room temperature. The peak PLemission wavelength and intensity increase with increasing well width.

Further, FIG. 3 illustrates the PL spectra of an a-planeIn_(0.03)Ga_(0.97)N/In_(0.1)Ga_(0.9)N MQW structure with a nominal wellwidth of 5.0 nm measured for various pump powers. PL intensity increaseswith pump power as expected while the peak emission wavelength is pumppower independent, indicating that the band profiles are not influencedby polarization-induced electric fields.

In addition to (In,Ga)N quantum wells, heterostructures containing(Al,Ga)N/GaN superlattices may also be grown on the non-polar(11{overscore (2)}0) a-plane GaN layers. For example, heterostructurestypically consist of two layers, most commonly (AlGa)N on GaN, toproduce an electrical channel necessary for transistor operation. Thethickness and composition of the superlattice layers may comprise, butare not limited to:

9 nm Al_(0.4)GaN barrier

11 nm GaN well

In one example, Block 110 was repeated 10 times to form a 10-periodAl_(0.4)Ga_(0.6)N/GaN superlattice that was terminated with a 11 nm GaNwell layer. The superlattice was grown via MOCVD at conditions similarto those employed for the underlying template layer: ˜1100° C. growthtemperature, ˜0.1 atm growth pressure, 38 μmol/min Al flow, 20 μmol/minGa flow, and 40,000 μmol/min N flow. The Al flow was simply turned offto form the GaN well layers. Successful growth conditions are notstrictly defined by the values presented above. Similar to the (In,Ga)Nquantum wells, the luminescence characteristics of the superlatticedescribed above indicate that polarization fields do not affect thestructure.

FIG. 4( a) shows a 2θ-ω x-ray diffraction scan of the 10-periodAl_(0.4)Ga_(0.6)N/GaN superlattice, which reveals clearly definedsatellite peaks, while FIG. 4( b) illustrates the PL spectra of thesuperlattice characterized in FIG. 4( a). The absence ofpolarization-induced fields was evidenced by the 3.45 eV (˜360 nm) bandedge emission of the superlattice. The band edge emission did notexperience the subtle red-shift present in c-plane superlattices.

REFERENCES

The following references are incorporated by reference herein:

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6. P. Lefebvre, J. Allegre, B. Gil, H. Mathieu, N. Grandjean, M. Leroux,J. Massies, and P. Bigenwald, Phys. Rev. B 59 (1999) 15363.

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Conclusion

This concludes the description of the preferred embodiment of thepresent invention. The following describes some alternative embodimentsfor accomplishing the present invention.

For example, variations in non-polar (Al,In,Ga)N quantum wells andheterostructures design and MOCVD growth conditions may be used inalternative embodiments. Moreover, the specific thickness andcomposition of the layers, in addition to the number of quantum wellsgrown, are variables inherent to quantum well structure design and maybe used in alternative embodiments of the present invention.

Further, the specific MOCVD growth conditions determine the dimensionsand compositions of the quantum well structure layers. In this regard,MOCVD growth conditions are reactor dependent and may vary betweenspecific reactor designs. Many variations of this process are possiblewith the variety of reactor designs currently being using in industryand academia.

Variations in conditions such as growth temperature, growth pressure,V/III ratio, precursor flows, and source materials are possible withoutdeparting from the scope of the present invention. Control of interfacequality is another important aspect of the process and is directlyrelated to the flow switching capabilities of particular reactordesigns. Continued optimization of the growth conditions will result inmore accurate compositional and thickness control of the integratedquantum well layers described above.

In addition, a number of different growth methods other than MOCVD couldbe used in the present invention. For example, the growth method couldalso be molecular beam epitaxy (MBE), liquid phase epitaxy (LPE),hydride vapor phase epitaxy (HVPE), sublimation, or plasma-enhancedchemical vapor deposition (PECVD).

Further, although non-polar a-plan GaN thin films are described herein,the same techniques are applicable to non-polar m-plane GaN thin films.Moreover, non-polar InN, AlN, and AlInGaN thin films could be createdinstead of GaN thin films.

Finally, substrates other than sapphire substrate could be employed fornon-polar GaN growth. These substrates include silicon carbide, galliumnitride, silicon, zinc oxide, boron nitride, lithium aluminate, lithiumniobate, germanium, aluminum nitride, and lithium gallate.

In summary, the present invention describes a method for formingnon-polar (Al,B,In,Ga)N quantum well and heterostructure materials anddevices. First, non-polar (11{overscore (2)}0) a-plane GaN thin filmlayers are grown on a (1{overscore (1)}02) r-plane sapphire substrateusing MOCVD. These non-polar (11{overscore (2)}0) a-plane GaN layerscomprise templates for producing non-polar (Al,B,In,Ga)N quantum welland heterostructure materials and devices.

The foregoing description of one or more embodiments of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

1. A method for forming a nitride semiconductor device, comprising: (a) annealing a substrate; (b) depositing a nitride-based nucleation layer on the substrate; (c) growing one or more non-polar a-plane gallium nitride (GaN) layers on the nucleation layer; (d) cooling the non-polar a-plane GaN layers under a nitrogen overpressure; and (e) growing one or more non-polar (Al,B,In,Ga)N layers on the non-polar a-plane GaN layers.
 2. The method of claim 1, wherein the substrate is an r-plane sapphire substrate.
 3. The method of claim 1, wherein the substrate is selected from a group comprising silicon carbide, gallium nitride, silicon, zinc oxide, boron nitride, lithium aluminate, lithium niobate, germanium, aluminum nitride, and lithium gallate.
 4. The method of claim 1, wherein the grown non-polar (Al,B,In,Ga)N layers comprise at least one quantum well.
 5. The method of claim 4, wherein the quantum well comprises an InGaN quantum well.
 6. The method of claim 4, wherein the quantum well is capped with GaN.
 7. The method of claim 1, wherein the grown non-polar (Al,B,In,Ga)N layers comprise at least one heterostructure.
 8. The method of claim 7, wherein the heterostructure contains an (Al,Ga)N/GaN superlattice.
 9. The method of claim 1, wherein the growing steps are performed by a method selected from a group comprising metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), hydride vapor phase epitaxy (HVPE), sublimation, and plasma-enhanced chemical vapor deposition (PECVD). 